1. Field of the Invention
The present invention is generally in the field of electronic circuits and systems. More specifically, the present invention is in the field of communications circuits and systems.
2. Background Art
Transceivers are typically used in communications systems to support transmission and reception of communications signals through a common antenna, for example at radio frequency (RF) in a cellular telephone or other mobile communication device. A transmitter routinely implemented in such a transceiver in the conventional art may utilize several processing stages to condition and preamplify a transmit signal prior to passing the transmit signal to a power amplifier (PA). For example, the transmit signal may originate as a digital signal generated by a digital block of the transmitter. The digital signal is then typically converted into an analog baseband signal, by means of a digital-to-analog converter (DAC) for instance. The analog baseband signal may then be filtered using a low-pass filter (LPF) and up-converted to RF by a mixer, which is usually implemented as an active circuit. Subsequently, the up-converted signal can be processed by a PA driver, which then passes the preamplified transmit signal to the PA for final amplification and transmission from the transceiver antenna
In a conventional transmitter preamplification chain, pre-amplification gain control, may be approximately evenly distributed between lower frequency gain control stages implemented prior to or in combination with up-conversion, and higher frequency gain control stages following up-conversion. In the conventional design approach, the DAC, LPF, and mixer circuits may collectively contribute a significant portion of the overall gain control, such as approximately fifty percent of the preamplification gain control, for example. However, this conventional approach is associated with significant disadvantages, owing in part to the substantial inefficiencies resulting from the time and iterative testing required to coordinate calibration amongst the various lower frequency and higher frequency gain control stages.
Additional disadvantages associated with conventional transmitter preamplification chain designs result from the inherent limitation imposed by their feedback stage architectures. For example, conventional designs result in feedback stages that are susceptible to oscillation during high gain operation. Moreover, conventional feedback stages often produce unreliable feedback data when the feedback signal includes local oscillator feedthrough.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing an improved transmitter preamplification chain enabling efficient preamplification gain control and feedback calibration.